In which config file is this setting stored so I can check the ISPConfig and actual (amavisd?) configuration?
In no file. amavisd is reading the information out of the database. Please see amavisd-new manual and there the sql setup part for details on how this work. ISPConfig uses the defaul amavisd sql table layout as descrobed in the amavisd configuration.
If you like to see in detail what amavisd is doing with a email, you can e.g. turn on debugging in amavisd conf file.